I'm curious too, since I often factor in how much of a commitment a game is when picking what to play next. I'm coming up blank myself, though. I don't think I've completed any RPG in fewer than 10 hours. Granted, I tend to take my time and am often a bit of a completionist. I've beaten some RPGs in 10-15 hours that could probably be beaten in under 10 if I just skipped the optional stuff. Some of the shortest I've beaten are Final Fantasy Mystic Quest (SNES), Arcana (SNES), Rhapsody: A Musical Adventure (PS1), End of Serenity (PSP), and Magic Knight Rayearth (GB).